1. Field of the Invention
The present invention relates to a reaction tube for a semiconductor process for performing a process on target objects under a vacuum atmosphere, and a heat processing apparatus using the reaction tube. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.
2. Description of the Related Art
FIG. 12 is a view showing an example of a vertical heat processing apparatus (vertical furnace) used in a semiconductor manufacturing system. This apparatus includes a vertical quartz reaction tube 101 having an exhaust port 105 on the lower side. When a predetermined heat process is performed, a wafer boat 145 with a number of semiconductor wafers (which may be simply referred to as wafers) W stacked thereon at intervals is loaded into the reaction tube 101, which is then airtightly closed by a lid 143a. Then, the interior of the reaction tube 101 is heated by a heater 102 while a process gas is supplied into the reaction tube 101. There are various types of gas supply structure for such reaction tubes. In this example, a vertically long narrow gas supply duct 106 is attached to the outer surface of the reaction tube 101, and holes 107 are formed in the wall of the reaction tube 101, so that a gas is supplied from the duct through the holes 107 into the reaction tube 101.
Heat processes performed in vertical heat processing apparatuses include processes performed under a vacuum atmosphere, such as CVD (Chemical Vapor Deposition), ALD (Atomic Layer Deposition), and annealing. The vertical heat processing apparatus described above is used to perform such heat processes.
The reaction tube 101 suffers a stress directed inward due to a pressure difference generated between the outside and inside when the pressure inside the reaction tube 101 is set to be a vacuum. Consequently, a stress concentration occurs at the junction between the sidewall and the closed end wall of the reaction tube 101, and may bring about a breakdown stress, which causes implosion of the reaction tube (the reaction tube 101 is broken inward due to a pressure difference between the outside and inside). In order to prevent this problem, the conventional reaction tube is structured such that the closed end wall has a domed shape projected outward to disperse the stress by the rounded corner of the domed shape.
When a heat process is performed in the conventional reaction tube 101, however, the heat of wafers W is discharged upward from the reaction tube 101 through a space 103 inside the domed shape, as shown in FIG. 13, and so the temperature of the wafers W is lowered near their center. Further, a process gas supplied from the sidewall of the reaction tube 101 does not swiftly flow as a whole into a process field 120 for processing the wafers W, but partly flows into the upper space 103. Since the upper space 103 has a domed shape and thus is wide, part of the process gas flowing therethrough has a lower flow velocity, and so this part stays in the upper space 103 for a longer time and is more decomposed, as compared with part of the process gas inside the process field 120. In this case, decomposed part of the gas more flows around the peripheral portion of wafers W near the upper space 103, of the wafers W held on the wafer boat 145. Consequently, the film formation rate is higher on the peripheral portion of these wafers W and deteriorates the planar uniformity on these wafers W. Further, where decomposed part of the gas more flows around the peripheral portion of wafers W near the upper space 103, the average film thickness on these wafers W becomes larger than that of the wafers W therebelow, and so the inter-wafer uniformity of the film thickness may be also deteriorated.
In order to solve the problems described above, there is a conventional heat processing apparatus including a heat insulation body disposed on a wafer boat to prevent heat from being discharged from wafers W on the upper side (Jpn. Pat. Appln. KOKAI Publication No. 2004-111715 (Patent Document 1: Paragraph No. 0030 and FIG. 1)). According to this heat processing apparatus of Patent Document 1, the heat insulation body prevents the temperature of wafers W on the upper side from being changed. However, a gas can still stay in the upper space, and so the inter-wafer uniformity of the film thickness may be deteriorated. Further, the boat needs to be longer by that much corresponding to the length of the heat insulation body, and thereby increases the size of the apparatus. Further, reaction product components deposited on the heat insulation body may be peeled off and generate particles due to the difference in the coefficient of thermal expansion and contraction between the reaction product components and heat insulation body. In addition, the heat insulation body may be broken due to a heat shock.